Mutual intelligibility:
* High mutual intelligibility: Speakers of the two varieties can understand each other with relative ease, even if they have regional accents or use different vocabulary. This is a strong indicator of dialects.
* Low mutual intelligibility: Speakers of the two varieties struggle to understand each other without significant effort. This is a strong indicator of separate languages.
* Intermediate intelligibility: This is the trickiest case, as speakers may understand each other to some extent, but there may be significant differences in grammar, vocabulary, or pronunciation that hinder communication.
Linguistic distance:
* Small linguistic distance: The two varieties share a significant amount of vocabulary, grammar, and pronunciation. This suggests they are closely related, possibly dialects.
* Large linguistic distance: The two varieties have many differences in vocabulary, grammar, and pronunciation. This suggests they may be separate languages.
Sociolinguistic factors:
* Social prestige and standardization: One variety may be considered more prestigious or "standard" than the other. This can influence perceptions of whether they are dialects or separate languages.
* Political and historical factors: Political boundaries, national identity, and historical events can influence how language varieties are perceived.
* Social and cultural differences: Significant cultural differences between speakers of two varieties can reinforce the perception of separate languages, even if there is high mutual intelligibility.
Examples:
* English dialects: American English and British English are considered dialects of the same language due to high mutual intelligibility and a shared history and linguistic base.
* Spanish dialects: Castilian Spanish, Argentinian Spanish, and Mexican Spanish are considered dialects of the same language due to high mutual intelligibility.
* Swedish and Norwegian: These languages are mutually intelligible to a large degree, but have distinct literary standards and are considered separate languages due to historical and political factors.
* Hindi and Urdu: These languages are largely mutually intelligible, but have distinct literary traditions and are considered separate languages due to religious and cultural factors.
Ultimately, the distinction between dialects and languages is not always clear-cut. It is often a matter of judgment based on the factors listed above. Linguistic and sociolinguistic research can provide valuable insights, but the final decision often rests on how language varieties are perceived and used by speakers and communities.